1. Field of the Invention
The invention relates to brazing filler metals having nickel-chromium-based alloys containing transition metals such as iron and molybdenum and certain metalloids. The alloys include one or more of nickel, chromium, iron, molybdenum, boron, phosphorus, and silicon, and are particularly useful for brazing metals at lower temperatures than prior art while increasing corrosion resistance.
2. Description of the Prior Art
Atomized brazing powders disclosed in the prior art, with high chromium concentration and with phosphorus as the major melting temperature depressant, require a binder to keep the powder in place as practiced conventionally. This binder has to be “burned off” during the brazing cycle, which creates hydrocarbon build-up on the interior of the furnace. This mix of binder and powder typically outgases during the brazing cycle, making it more likely to trap gas pockets in the brazed object.
Prior art chromium-containing brazing foils have high concentrations of metalloids such as boron and phosphorous, which create brittle intermetallics in the brazed joints. These intermetallics are typically chromium borides or other chromium phosphides. Because of the brittleness of the brazed joints, the joints often crack, resulting in disintegration of the brazed products.
Brazing of nickel-chromium based alloys disclosed in the prior art is restricted to a vacuum-type oven because of the high temperatures that are required during the brazing operation, combined with the need for protective atmospheres. The vacuum brazing process is extremely slow and expensive as compared to a belt/continuous furnace. However, the brazing temperatures of powder and other brazing foils are too high for belt ovens, which are typically limited to 1100° C.